Optical disk drives use a laser diode for generating a laser beam, which is focused by an optical arrangement onto an optical disk for scanning a substantially circular track on the optical disk. The substantially circular track usually has the form of a spiral. For a read-only disk, the track may be a pre-embossed sequence of marks, which may also be referred to as pits, and spaces. The marks and spaces correspond to data stored on the optical disk. For a recordable disk, the track may be a substantially continuous groove, separated by land portions between subsequent turns of the spiral. The recordable disk may allow to write marks in the groove in a recordable layer. Write-once recordable disks allow to write marks, but do not allow to erase marks and overwrite them with new marks. Rewritable recordable disks, also referred to as re-recordable optical disks, allow to write marks, as well as erase marks and overwriting with new marks.
For writing marks on the optical disk, the optical disk drive may be equipped with a laser driver for applying sequences of drive pulses to the laser, resulting in high-power laser pulses, allowing e.g. to form the marks in the recordable layer. The accuracy of writing these marks is dependent on the accuracy of the actually applied laser power. When an incorrect or insufficient laser power is applied, mark formation will not be achieved as intended, e.g. all marks may become too short, and no correct retrieval of data from the disk will be possible. As the output power of a laser diode may vary e.g. due to a varying temperature, the output power of the laser diode is usually monitored and controlled. The optical disk drive usually comprises a sensor for sensing the laser diode output power of the laser beam and adjusting the laser diode output power to achieve a stable laser diode output power when writing marks.
Also when scanning the optical disk for reading the marks and spaces and retrieving the associated data from the marks and spaces, the laser diode output power is usually controlled. The laser diode output power usually shows a threshold current level below which the laser diode output power is marginally and above which the laser diode output power increases strongly, and reading is usually performed with a laser diode output power only closely above to the threshold current level, it is common to also monitor and control the laser diode output power during reading and to thus provide a substantially stable laser diode output power during reading. This power may also be referred to as the read output power.
The monitor and control of the laser output power normally results in a well-defined laser output power, substantially independent of e.g. the temperature of the laser diode and possible other effects effecting the relationship between laser diode output power and laser driving conditions, e.g. laser driving current. However, the monitor and control will not always be able to give a well-defined laser output power. E.g., the driving current may have increased to a level at which the laser diode is damaged and the laser diode is not able to deliver the required output power anymore, resulting in badly recorded disks which cannot be read with a sufficiently low number of errors. Also, the damage to the laser diode may result in e.g. non-linearity of the laser diode output power as a function of driving current, causing the control loop to fail to control sufficiently accurate.
Recording quality may thus decrease and recording with a sufficient quality for correctly reading back the recorded data may eventually fail when the laser diode has degraded. Although an optical disk may still appear be readable immediately after recording with the optical disk drive used for the recording, also when the recording quality is poor, the same optical disk may not be readable in other optical disk drives when the recording quality is poor. Especially in applications where a large value is associated with a recorded optical disk, this may be unacceptable. E.g., when an optical disk is used for personalized sale, where e.g. a specific compilation of content, such as movies, is recorded on a recordable optical disk in a shop and sold for a price similar to that of a pre-embossed read-only disk, the buyer will expect the same quality for the personalized, recordable optical disk as for the pre-embossed read-only disk. Also in archiving applications, either professional or for personal use, the quality of the recorded disk shall be high, and allow reading back the disk correctly on a wide variety of optical disk drives, also at a much later moment in time. A similar requirement is set by disk duplicator farms, which produce copies of a master optical disk by copying them with a farm of optical disk drive on recordable optical disks. Another example is with a so-called Medical Grade Disk, which is an optical disk carrying patient data, e.g. digital X-ray photographs, which need to be reliable recorded and correctly read.
Some known methods test each recorded disk on the quality of the recorded marks. Although this may be a viable solution for some applications, it may be quite cumbersome for others. Improved methods thus aim to prevent recording when the laser diode has degraded, instead of repeating the recording on another optical disk drive when it has been detected that a recording has a poor quality, or even has failed.
A known method aiming to prevent the use of optical disk drives with a degraded performance is to replace an optical disk drive after a pre-determined number of operating hours or a pre-determined number of recording sessions, at which the optical disk drive is expected to still be of sufficient quality. Although this increases the number of correct recordings, the known method has the drawback that still bad recording may arise from optical disk drive that fail earlier than expected. Also, optical disk drives which still function perfectly are replaced, while they might have been used for considerably longer before the recording quality deteriorates.